The present invention relates to a method for producing an air-quench-toughened glass plate, and more particularly to a method for producing a high-quality air-quench-toughened glass plate including a step for compulsorily breaking toughened glass containing nickel sulfide (NiS), to thereby remove defective products.
Air-quench-toughened glass (float glass) plates, particularly such plates having a soda-lime composition, have widely been employed as panes in buildings and as side and rear windows of automobiles. In a conventional method for producing soda-lime glass, in a step for melting glass raw materials at a high temperature in the vicinity of near 1,500xc2x0 C. in a melting furnace, a nickel (Ni) component contained in stainless steel used for the interior of the melting furnace and Ni-containing metal particles (e.g., stainless steel particles) contained as impurities in a glass raw material may be mingled into molten glass, and the Ni component may react with a sulfur (S) component contained in mirabilite (Na2SO4) serving as a glass raw material. As a result, nickel sulfide (NiS) may be present in a melt-molded glass substrate. The incidence of an NiS impurity in a defective glass product is very low; i.e., the number of impurities is about one in some 10 tons (t) of glass products. In addition, the impurity has a spherical shape and the particle size is as small as 0.3 mm or less, and thus detection of the impurity in a production line is very difficult.
In order to process such substrates formed of soda-lime glass into glass products used in buildings and automobiles, glass substrates are toughened by heating to the softening point of glass (about 620xc2x0 C.) and quenching to about 450xc2x0 C. by means of an air-jet (a so-called quenching step), to thereby generate compressive stress (e.g., 100 kg/cm2 or more) in the surfaces of the resultant glass plates. This process is called air-quench toughening.
When nickel sulfide (NiS) is present as an impurity in air-quench-toughened glass which is heated and cooled to ambient temperature in a toughening step, xcex1-phase NiS, which is stable at about 350xc2x0 C. or higher, is present in an unstable state. Since xcex1-phase NiS is unstable at ambient temperature, with passage of time it is transformed into xcex2-phase NiS, which is stable at ambient temperature. The volume of NiS increases concomitant with phase transformation. A toughened glass plate contains a tensile stress layer having a thickness which is about ⅔ the overall thickness of the plate, and thus cracks (breakage) rapidly grow due to an increase in the volume of NiS in the tensile stress layer, to thereby cause spontaneous breakage of the glass plate.
In recent years, these air-quench-toughened glass plates having a large surface area have predominantly served as a building material, and toughened glass products have had a thickness up to 4-19 mm and considerable weight. Thus, as a measure against spontaneous breakage of toughened glass plates induced by nickel sulfide (NiS) impurities or other glass defects contained in toughened glass plates, protective film formed of an organic material is laminated on either side of a toughened glass plate, to thereby prevent the glass plate from becoming a hazard in the event of breakage.
However, a conventional method including lamination with protective film does not serve as an essential measure for preventing spontaneous breakage of toughened glass plates induced by phase transformation of nickel sulfide (NiS).
In addition to the above-described method, there is known a method for removing defective products containing NiS impurities, which method comprises inserting in a firing (soaking) furnace a toughened glass plate which has been heated during a toughening step and annealed to ambient temperature; heating again to a predetermined temperature (typically 290xc2x0 C. or higher), and maintaining the temperature for a predetermined time to thereby cause phase transformation of NiS from unstable xcex1 phase (xcex1 phase is stable at about 350xc2x0 C. or higher) to xcex2 phase, which is stable at 290xc2x0 C. or higher, concomitant with an about 4% expansion in volume; and compulsorily breaking any NiS-containing, defective toughened glass plate through drastic growth of generated cracks. This method is called a batch-manner soaking process.
In such a batch-manner soaking process, a toughened glass plate containing NiS must be broken, to thereby reliably remove any defective product.
A conventional batch-manner soaking process requires a long time and great amount of thermal energy for elevating temperature, since a toughened glass plate which had once been cooled to ambient temperature is heated again to a predetermined temperature. In addition, the time during which the glass plate must be maintained at a predetermined temperature varies with the thickness of the glass plate, to thereby elevate production cost for glass plates.
Furthermore, in a conventional batch-manner soaking process, the phase transformation of nickel sulfide depends considerably upon the temperature elevation rate. Therefore, a variety of conditions must be investigated so as to determine optimum operational conditions.
In addition, in a conventional batch-manner soaking process, when the composition or thickness of a glass plate during temperature elevation is altered, conditions for causing phase transition of nickel sulfide (predominantly maintenance temperature and time) vary. Thus, removal of all toughened glass products containing nickel sulfide is difficult.
An object of the present invention is to provide a method for producing an air-quench-toughened glass plate including conditions for compulsorily and reliably breaking defective NiS-containing glass during a conventionally-employed batch-manner soaking process.
Another object of the present invention is to provide a method for producing an air-quench-toughened glass plate comprising a continuous-manner soaking process.
Another object of the present invention is to provide a method for producing an air-quench-toughened glass plate comprising compulsorily and reliably breaking defective NiS-containing glass without employing a conventionally-employed batch-manner soaking process.
Still another object of the present invention is to provide an air-quench-toughened glass plate produced through any of the above methods.
According to a first mode of the present invention, there is provided a method for producing a toughened glass plate comprising the steps of:
carrying out air-quench toughening for quenching a glass plate heated to near a softening point thereof to thereby generate compressive stress in the surface layers of the glass plate;
lowering the temperature of the toughened glass plate to ambient temperature; and
elevating the temperature of the glass plate from ambient temperature to a predetermined temperature range to thereby cause phase transformation of nickel sulfide (NiS) contained as a molten impurity in the glass plate from xcex1-NiS (xcex1 phase) to xcex2-NiS (xcex2 phase) concomitant with expansion of the volume thereof, resulting in compulsory breakage of the glass plate to thereby remove defective products, characterized in that the predetermined temperature range and a temperature elevation rate to the range satisfy the following conditions:
when the temperature elevation rate is about 3xc2x0 C./minute, the predetermined temperature range is 170-320xc2x0 C.;
when the temperature elevation rate is about 5xc2x0 C./minute, the predetermined temperature range is 180-320xc2x0 C.;
when the temperature elevation rate is about 6xc2x0 C./minute, the predetermined temperature range is 185-325xc2x0 C.;
when the temperature elevation rate is about 10xc2x0 C./minute, the predetermined temperature range is 215-340xc2x0 C.;
when the temperature elevation rate is about 20xc2x0 C./minute, the predetermined temperature range is 235-345xc2x0 C.;
when the temperature elevation rate is about 40xc2x0 C./minute, the predetermined temperature range is 270-350xc2x0 C.; and
when the temperature elevation rate is about 50xc2x0 C./minute, the predetermined temperature range is 285-350xc2x0 C.
According to a second mode of the present invention, there is provided a method for producing a toughened glass plate comprising the steps of:
carrying out air-quench toughening for quenching a glass plate heated to near a softening point thereof to thereby generate compressive stress in the surface layers of the glass plate; and
in the course of carrying out annealing following the air-quench toughening, maintaining toughened glass containing nickel sulfide (NiS), which causes growth of cracks along with residual thermal stress, at a predetermined temperature for a predetermined time to thereby cause phase transformation of the nickel sulfide (NiS) from xcex1-NiS xcex1 phase) to xcex2-NiS (xcex2 phase) concomitant with expansion of the volume thereof, resulting in drastic growth of cracks and simultaneous breakage of the glass.
According to a fifth mode of the present invention, there is provided a method for producing a toughened glass plate comprising the steps of:
carrying out air-quench toughening for quenching a glass plate heated to near a softening point thereof to thereby generate compressive stress in the surface layers of the glass plate; and
in the course of carrying out annealing following the air-quench toughening, annealing toughened glass containing nickel sulfide (NiS), which causes growth of cracks along with residual thermal stress, at a temperature from near 300xc2x0 C. to 150-200xc2x0 C. at a rate of less than 12xc2x0 C./minute to thereby cause phase transformation of the nickel sulfide (NiS) from xcex1-NiS (xcex1 phase) to xcex2-NiS (xcex2 phase) concomitant with expansion of the volume thereof, resulting in drastic growth of cracks and simultaneous breakage of the glass.
According to a third mode of the present invention, there is provided a method for producing a toughened glass plate including a production step for a glass plate comprising a melting step, a shaping step, and an annealing step, immediately followed by a toughening step, characterized in that the method comprises the steps of:
in the course of carrying out the annealing step, lowering the temperature of the glass plate to a predetermined temperature range and maintaining the glass plate at a predetermined temperature within the temperature range of 160-280xc2x0 C. for 6-30 minutes to thereby cause phase transformation of the nickel sulfide (NiS) contained in the glass plate from xcex1-NiS (xcex1 phase) to xcex2-NiS (xcex2 phase) concomitant with expansion of the volume thereof, which generates cracking around nickel sulfide particles; and
in a toughening step following the annealing step, quenching the glass plate from near the softening point thereof to thereby generate compressive stress in the surface layers of the glass plate and, subsequently, causing drastic growth of cracks to thereby compulsorily cause breakage the glass plate so as to remove defective products.
According to a fourth mode of the present invention, there is provided a method for producing a toughened glass plate including a production step for a glass plate, immediately followed by a toughening step, characterized in that the method comprises the steps of:
in the course of carrying out a pretreatment step prior to the toughening step, heating the glass plate to thereby cause phase transformation of the nickel sulfide (NiS) contained in the glass plate from xcex1-NiS (xcex1 phase) to xcex2-NiS (xcex2 phase) concomitant with expansion of the volume thereof, which generates cracking around nickel sulfide particles; and
in a toughening step following the pretreating step, quenching the glass plate from near the softening point thereof to thereby generate compressive stress in the surface layers of the glass plate and, subsequently, causing drastic growth in cracks to thereby compulsorily cause breakage of the glass plate so as to remove defective products, and the predetermined temperature range and the temperature elevation rate to the range employed in the pretreatment step satisfy any one of the following conditions:
when the temperature elevation rate is about 3xc2x0 C./minute, the predetermined temperature range is 170-320xc2x0 C.;
when the temperature elevation rate is about 5xc2x0 C./minute, the predetermined temperature range is 180-320xc2x0 C.;
when the temperature elevation rate is about 6xc2x0 C./minute, the predetermined temperature range is 185-325xc2x0 C.;
when the temperature elevation rate is about 10xc2x0 C./minute, the predetermined temperature range is 215-340xc2x0 C.;
when the temperature elevation rate is about 20xc2x0 C./minute, the predetermined temperature range is 235-345xc2x0 C.;
when the temperature elevation rate is about 40xc2x0 C./minute, the predetermined temperature range is 270-350xc2x0 C.; and
when the temperature elevation rate is about 50xc2x0 C./minute, the predetermined temperature range is 285-350xc2x0 C.